A number of explosion-proofing protective measures are available for preventing unwanted and dangerous explosions of explosive gas mixtures in hazardous areas. These include an “intrinsic safety” protective measure. According to this protective provision, electric lines that lead into areas exposed to explosion hazards cannot create a spark whose energy is sufficient to ignite the gas mixture. Accordingly, it needs to be ensured that the voltage in the line and the maximum current being transmitted remain within permissible limits.
This can be enforced with so-called safety barriers. These safety barriers usually feature an ohmic resistor or transistors in the series arm. Due to their circuit design, however, they are usually not suitable for the power supply of modern bus systems.
Another disadvantage of these safety barriers is that they usually contain components that are irreparably destroyed when certain limit values are exceeded. Consequently, it is not only required to replace the device that has caused the damage, but also the safety barrier. This naturally results in a significant loss of time during the operation of the system protected by the safety barrier.
Another problem arises if semiconductors that today usually consist of normally off field-effect transistors are used in the series arm. In particular, under certain circumstances, a thermally overloaded semiconductor can no longer be properly switched off via its control input.
In other known applications, auxiliary voltages are generated from the supplied electrical energy by means of transformers in the hazardous zone. If such a transformer fails and delivers an excessive voltage, it is no longer possible to intervene with classic safety barriers—the excessive voltage occurs downstream of the safety barrier and not at its input.
In addition, known safety barriers interrupt the electrical connection between input and output, but are usually unable to short-circuit damaging voltages returning from the hazardous zone at the output.
Based on these circumstances, the invention aims to develop protective circuit arrangements that according to their design solve one or more of the aforementioned problems.